1. Field of the Invention
The present invention relates to a compact zoom lens, and more particularly to a zoom lens whose variable power ratio is about 8 and F number is about 1.2 suitable for a video camera and a still video camera.
2. Description of the Prior Art
With recent rapid popularization of video cameras, particularly VTR integrated video cameras or still video cameras, these cameras have been tried to be compact. A zoom lens mounted on these cameras has also been desired to be compact and to have a high variable power ratio and a large aperture. Furthermore, a combination of a camera (image pickup) element such as a CCD and a color filter is used for formation of color signal. Therefore, luminous flux incident thereon is desirable to be incident on an image pickup surface substantially vertically to avoid mixing of colors caused by a color filter. An optical system used here is desired to be a telecentric structure in which an exit pupil is substantially infinitely far from an image pickup surface. It is also necessary to have enough backfocal length to install a lowpass filter for preventing spurious signal due to a spacial frequency beyond resolution of an image pickup element, an infrared cut filter to restrict a spectral sensitivity of the element within a visible area and the like.
When a diameter and a total length of a zoom lens are tried to be reduced maintaining a high variable power ratio and a large aperture and fulfulling various conditions as described above, it becomes difficult to correct various aberrations such as distortion, comatic aberration, chromatic aberration and the like, failing to satisfy such an imaging performance as desired.
Zoom lenses having a high variable power ratio which are bright in which a variable power ratio is 8 and F number is about 1.2 for video cameras and still video cameras have been heretofore known from
(1) Japanese Patent Application Laid-Open Publication No. 222,807/1984 PA1 (2) Japanese Patent Application Laid-Open Publication No. 186,818/1985 PA1 (3) Japanese Patent Application Laid-Open Publication No. 93,423/1986 PA1 (4) Japanese Patent Application Laid-Open Publication No. 54,213/1987 PA1 R.sub.44 : Radius of curvature of a surface on the image side of a positive lens closest to the image side in the fourth lens group PA1 f.sub.5 : Synthesized focal length of the fifth lens group PA1 f.sub.51 : Synthesized focal length of the front unit in the fifth lens group PA1 AN.sub.52p : Average value of the refractive indices of the positive lenses in the rear unit of the fifth lens group PA1 R.sub.5 : Radius of curvature of the surface on the image side of a negative meniscus single lens in the rear unit of the fifth lens group PA1 AV.sub.2N : Average value of Abbe's number of the negative lenses in the second lens group PA1 V.sub.2p : Abbe's number of the positive lens in the second lens group PA1 Z: Variable power ratio PA1 F.sub.w : Open F number at the wide end of the entire zoom lens system
and the like.
Zoom lenses having a high variable power ratio whose variable power ratio is about 12 have been known from Japanese Patent Application Laid-Open Publication No. 71,015/1984 and the like.
These zoom lenses each comprise, from an object side, a first lens group having a positive refracting power, a second lens group having a negative refracting power and being movable to vary the power, a third lens group having a negative refracting power and being movable to compensate for deviation of an image position caused by varying the power, a fourth lens group having a positive refracting power and making a divergent luminous flux which exits from the third lens group substantially afocal, and a fifth lens group having a positive refracting power for forming an image.
The aforesaid lens is a relatively compact system without complicating a construction of a lens-barrel and has been heretofore widely used. However, a length (hereinafter referred to as total length) from a surface of a lens closest to the object to an image pickup surface is as large as 14 to 16 times of a focal length at the wide angle end, and with this, a diameter of a front lens becomes large, which cannot be said sufficiently compact.
Moreover, the lens having high variable power ratio such as about 12 is slow or dark such as a F number of 1.6 at the wide angle end, which cannot be said to have a sufficient brightness for a video camera or a video still camera.